Need Help On Cake Size

Hello - I am making a graduation cake and wondering what would be the best size to feed about 30-45 people. They're not sure how many are going to show up, although they sent out invitations to about 75 people! My preference is to do square or round tiers, my sheet cakes never seem to be level. Would a 12 inch and 8 inch be okay?Thanks for your help! Ruth